This invention relates to a projection display apparatus that enlarges an image displayed on an image source such as a cathode ray tube (CRT), and projects it onto a screen. More particularly, this invention relates to an apparatus that can project a finely detailed, high contrast image on the screen.
FIG. 21 is a schematic diagram showing the construction of the image projecting part of a projection display apparatus described in the Japanese Patent Kokai Publication No. 258575/1994 (applicant: Mitshubishi Denki Kabushiki Kaisha). As shown in FIG. 21, the image projecting part comprises a CRT 3 having a faceplate 3f, a projecting lens assembly 4 comprising six lens elements (or lens groups) L.sub.1 -L.sub.6, and a cooling fluid 6. In FIG. 21, S.sub.1 -S.sub.12 indicate surfaces of each lens element L.sub.1 -L.sub.6. In this image projecting part, a spot displayed at the center A of the faceplate 3f, through which an optical axis 5 of the projecting lens assembly 4 passes, is projected as a spot image, which almost perfectly resembles the image on the faceplate 3f, at the center A.sub.s of a screen as shown in FIG. 22.
However, in the aforesaid conventional projection display apparatus, due to off-axis aberrations such as coma or astigmatism, and to discrepancies when machining and assembling the lenses, a spot displayed in a corner B of the faceplate 3f of the CRT 3 is projected on the screen so that the projected image on the screen has the deformed and widened form shown as B.sub.s in FIG. 22. This causes the resolution of a projected image to decrease. Moreover, flare affects adjacent images, leading to local decrease of contrast of the projected image on the screen.
In a model representation of the image formed in the area B.sub.s of FIG. 22, two regions may be distinguished as shown in FIG. 23, viz. a region 31 where the light beam is relatively converged, and a region 32 where the aberration light (referred to hereinafter as flare) surrounding the region 31 is impinged. According to this model, the light intensity distributions in the meridional and sagittal directions are respectively 33M and 33S. The MTF (modulation transfer function) is obtained as the Fourier transform of the intensity distribution. Assuming the basic frequencies to be the same, the characteristics of the MTF 34M in the meridional direction are then superior to those of the MTF 34S in the sagittal direction, as shown in FIG. 24.
The Japanese Patent Kokai Publication No. 251808/1990, for example, proposes a means of overcoming this problem. According to this proposal, flare is screened out by providing an obstructing means in the projecting lens assembly. However, as the position of this obstructing means is not specified, it may obstruct light except for the flare, leading to a decrease of brightness of the projected image.